The objective of this research is to investigate the DNA damage and repair of irradiated tumor cells at different phases of the cell cycle (G1, S, and G2). DNA repair kinetics will be measured after low doses of radiation. The results will be compared to the survivals of irradiated tumor cells to determine if the relative rate or extent of DNA repair is responsible for the variations in cell survival during different phases of the cell cycle. 9L rat brain tumor cells and EMT-6 mouse mammary tumor cells will be synchronized in situ and in vitro by centrifugal elutriation. The relative rate and extent of repair of radiaton-induced DNA strand breaks, DNA-protein crosslinks, and DNA-DNA crosslinks will be measured by the alkaline elution assay. Colony forming assays will determine the radiaton sensitivity of these cells throughout the cell cycle. CHO cells will be used for all experiments as a control to evaluate the differences between "normal" cells and tumor cells and the differences between centrifugal elutriation and other synchrony methods. Direct comparison of in situ and in vitro studies will be used to determine if the in vitro models are appropriate for extrapolation to solid tumors. Variations in cell survival at various phases of the cell cycle may result from changes in the chromatin template or in the quantity or function of the repair enzymes in irradiated cells. These possibilities will be examined by measuring at different phases of the cell cycle the activity of the alpha and beta polymerases and the suitability of isolated chromatin as a template for nuclease and polymerase activity. These studies of the relationship between cellular radiation response and DNA damage and repair both in situ and in vitro as a function of position in the cell cycle could provide not only information on the molecular mechanisms of DNA repair processes in mammalian cells, but also a better rationale for the design of cancer treatments.